Motor Failure - Reciprocating compressor on VFD 1

[nhcf]

Does anyone have good reference material on application of VFD on reciprocating compressor loads.

Existing facility experienced a motor insulation failure on a 150 HP 480 VAC compressor. It is not clear how much engineering was applied to the initial conversion of the compressor to VFD control, so I am trying to familiarize myself with the various considerations, to confirm whether improper controls/configuration could have contributed to the failure.

 

[John2025]

Is there an advantage to a VFD on a recip? Doesn't it just unload and shutdown when it's not under load?

 

[ScottyUK]

How old is the drive motor? Is the winding insulation adequately rated or reinforced for use with a VFD?

 

[itsmoked]

John; It prevents start stops which are arguably the biggest wear generator.

nhcf; VFDs put large voltage transients onto motor windings (often thousands a second) if this was "a conversion" it could be the motor's insulation system was not up to par for VFD duty - a common occurrence.

Where exactly did the insulation fail?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[nhcf]

Thanks for replies.
We are still attempting to gather information on the application. The motor failure was determined to be insulation failure, although the motor was inverter-duty. There are other potential issues associated with the installation, including length of motor leads/non-vfd cable that have likely contributed. I have not applied VFD to recip load before and am reading posts suggesting mechanical issues with running at reduced speeds on VFD (lubrication, vibration) and I am trying to determine if the recip load could contribute to failure.

 

[ScottyUK]

At low speed the compressor flywheel becomes less effective as an energy store and the torque seen by the motor will vary cyclically through each rotation. Pulsating torque means pulsating current, which in turn raises the possibility of winding movement if the taping and bracing isn't very tight and/or if the resin impregnation is imperfect. Lubrication could be a problem if the machine has babbit bearings because the oil wedge won't establish itself properly in the bearing shell if the speed is too low, but on a machine this size I imagine they're rolling element.

Don't often hear of VFD's on recip compressors. Maybe there's a reason for that?

 

[jraef]

ScottyUK has hit the nail on the head. VFDs on recips often create more problems that they solve. People often are misguided as to the potential energy savings because they see savings with pumps, fans and even CENTRIFUGAL compressors. But there is no panacea of energy savings on a constant torque load like a reciprocating compressor. Yes, Keith is correct it will help out with the starting and stopping shock load, but a soft starter will do that too, and not bring with it the potential problems posed by VFDs.

Motor winding insulation damage is absolutely a potential issue when using VFDs, especially on older motors made with 1000V insulation. bearing EDM damage is another one, plus all the other things ScottyUK mentioned. Without the big incentive of energy savings that are not going to be true on a recip, there is little justification for using it at all. Now that the motor must be changed, it might be a good time to investigate why the VFD was used in the first place and if it was because of a perceived energy savings scheme, this might be the time to challenge the assertion.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[nhcf]

ScottyUK and jraef thanks for the responses. Good input and reinforces my thinking that the VFDs may have been misapplied to begin with for this recip application. I'll need to perform some more research at the site to understand the history and basis.

 

[waross]

It may be OK to use a VFD for starting but you don't want to slow the machine down.
As Scotty says you will lose the flywheel effect and get large torque pulsations.
The torque pulsations will be accompanied by current pulsations.
The I²R heating will probably drive the RMS loading past the safe motor loading point.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Compositepro]

A VFD or soft starter would probably not work for starting because the starting torque will be higher than the running torque without the flywheel assist. Reciprocating compressors start unloaded, either by an unloading valve or, in the case of refrigeration compressors, a time delay to let the high pressure bleed down before starting again.

 

[GroovyGuy]

Howdy,
We have installed lotsa ASDs on recip compressors over the last several years. This was done as much for process reasons (ie turn-down) as for the ability to start and accelerate the compressor on weak power systems. These compressors have varied in size from a few hundred hp to 7000hp. Recently we have implemented synch-transfer on many of the larger units. For MV applications (ie 4000V thru 6600V motors) we generally use API-541 motors, although sometimes we can get away with a API-547 motor. For LV applications (ie 460V or 575V motors) we always use a IEEE-841 motor.
In most circumstances, the motor is started with the compressor unloaded.

The key to having a successful installation is getting the right motor.
I have a few questions wrt your problem;
1) What type of motor (ie is it a IEEE-841)?
2) If not an IEEE-841, is the motor Inverter-duty rated?
3) Is the motor designed for a constant-torque application over the desired down-down ratio?
4) Is the motor fitted with an external blower fan? This is almost always provided on a Constant Torque application where the turn-down is > 2:1.
5) Who spec'd and supplied the motor (ie was it the compressor OEM)?
6) What is make / model of the motor?
7) What is the length of the motor feeder cable?
8) Was the drive fitted with an output dv/dt filter?

GG

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

 

[LionelHutz]

Having a motor with "inverter duty rated" on it doesn't necessarily mean much. I know of at least one manufacturer that just sends someone out to the shop to drill and rivet an "inverter duty rated" plate onto a standard motor when inverter duty is ordered.

I would be expecting the system to experience surging that would be overloading the motor during each compression stroke when running at a reduced speed, due to the energy storage of the flywheel depending on rpm^2. I suppose you could design the compressor to operate over a speed range with an oversized flywheel? It really doesn't sound like a great application to just throw a VFD onto.

 

[moltenmetal]

We just say no to VFDs on positive displacement compressors of any kind. The machines are designed to run in a narrow range of generally fixed speeds. The turn-down ratio for such compressors on speed is very low, for the reasons ScottyUK and others have identified. If you need to unload, you need valve unloaders.

 

[nhcf]

Thanks for the feedback -
- The failed motor was WEG, 'crusher-duty' motor, inverter duty but I don't see IEEE 841 referenced. The compressor mfr/oem was not involved in the VFD integration. VFD is an AB Powerflex 400, lead lengths are approx. 200 ft, single conductors in conduit. No load side filters.

My sense is this is a misapplication of the VFD control, but still chasing some other aspects of the install.

 

[GroovyGuy]

moltenmetal; I agree with you that you likely don't require a VFD for every recip-compressor that comes your way, but seriously I have installed dozens of VFDS on recips over the last few years and have had no issues. These are not small machines either, likely averaging several thousand hp per machine.

nhcf: WEG is the quality of motor that you get when you don't spec something better (ie Toshiba, Siemens, Baldor).
A couple of things to watch for;
1) Turn down ratio (if > than 2:1, you will require an external blower motor)
2) Output dv/dt filters; (always a good idea on any drive, regardless of the cable length)
3) Special drive cables (waste of money if you have dv/dt filters)
4) Ensure that the motor OEM is aware of the application (ie constant-torque recip-compressor, turn-down ratio, operating speed range, type of drive, etc)

GG


"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

 

[waross]

A multi cylinder compressor (6 cylinedrs or more) won't be as bad as a single or twin cylinder for torque surging.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

True Bill.

Often the difference of a hundred RPM can preclude all start-stops of the compressor (short-cycling). This is way better for the motor than the torque ripple inherent in a few hundred RPM off of nominal.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ScottyUK]

GG - interested what features you specified under API-541 for this duty. Buying one isn't a straightforward case of "I want an API-541 motor" - there are a *lot* of options and customer choices under that standard. API-547 is a lot simpler. Would you happen to have an API summary sheet for one which you could post? I may have a similar compressor and motor being installed in the next year or two.

 

[LionelHutz]

WEG - The motor involved in more than one finger pointing dance I've been sucked into over the years. The need to finger point ends when a different motor is installed or a shop does a proper re-wind of the failed motor.

Also, see my last post....

 

[GroovyGuy]

Hi Scotty,
The following flow-chart is an example of the decision making process that one of our Clients will use to determine the selection process between a API-541 and API-547 motor. [ BTW this Client has hundreds of compressors installed, and installs at least a dozen (or more) per year. All are on ASDs. Most are API-541 motors. ]

As you can well imagine, with this flow-chart, not many motors end up being a API-547 motor. Recently they have veered away from this flow-chart if there is a 100% spare installed.
The API-541 motor is considerably more costly than a API-547 motor, more so depending upon the level of testing required.
GG

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)